A column of this type is represented in a traditional way in FIG. 1. It includes an enclosure 1 comprising a wash zone A at which a liquid wash flow L is introduced. Beneath the wash zone A there is an overflash zone B, where a two-phase mixture M is introduced, composed of gas and liquid which is separated into an ascending gaseous flow V and a liquid phase L′ flowing by gravity toward the lower area of the enclosure. This overflash zone B is provided with a flow device 2 formed from an appreciably horizontal collector tray 3 having openings 4 for the passage of the ascending gaseous flow V and descending liquid flow L. The flow device also comprises a channel 5 for recovering the descending liquid flow L, which channel has a slight slope with respect to the tray. The channel 5 discharges into a flow orifice 6 allowing the flow of the descending liquid flow L to the lower part of the column. This flow orifice 6 is usually connected to an appreciably vertical tubular member 7 (liquid flow draw-off tubular member), inside of which the liquid L flows. The collector tray is provided either with a single channel or multiple channels, each discharging into a single orifice. Because the walls of the tubular member 7 are heated by the ascending gaseous flow, the draw-off liquid flow, flowing inside the tubular member in a thin layer on its internal walls from the flow device 2, is subject to a high temperature during the time of contact (the slower the flow speed, the higher the temperature), which causes the formation of coke on the walls by thermal cracking of the flowing liquid. The accumulation of coke on the internal walls of the tubular member 7 creates a growing obstruction of its passage and ends by causing the blockage thereof, thus requiring the shut-down of the facility. Moreover, the flow descending in a thin film of liquid along the inner walls of the tubular member can be slowed by the ascending gaseous flow, which increases the time the liquid spends on the hot walls of the tubular member and worsens the coking phenomenon.
The coking phenomenon is thus promoted by the heat exchanges between the ascending gaseous flow and the descending liquid flow, said heat exchange increasing as the heat exchanging surface and the time the liquid flow spends in the tubular member are increased. This coking phenomenon is therefore also promoted both by prolonged contact between the ascending gaseous flow and the descending liquid flow, and by prolonged contact between the liquid flow and the inner wall of the tubular member—the slower the speed of the descending liquid flow, the longer the contact.
The coking of this flow device (at the tubular member 7 or the orifice 6) is a serious hindrance, because it not only steadily increases the loss of load of this overflash zone and reduces the hydraulic capacity of the collector tray, it also reduces the cycle times of a facility because it is necessary to shut down the facility prematurely to remove the coke by mechanical means.
Increasing the speed of the film flowing on the inner walls of the tubular member would make it possible to limit the coking phenomenon, because the time the liquid spends on the hot walls of the tubular member would be reduced. One method of increasing this speed would consist of decreasing the diameter of the tubular member. However, such a decrease is contrary to the current development of the technology, which tends to increase the diameter of the tubular member in order to slow as much as possible the blocking of the tubular member due to the formation of coke.